This invention is directed to a process for preparing a polyester or a poly(ester carbonate) which process comprises reacting a dihydric phenol with an aromatic diacid and a diaryl carbonate in the presence of from about 10 to about 60 weight percent, based on the weight of the polyester produced, of a processing aid under polyester or poly(ester carbonte) forming conditions.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl) propane, also identified as Bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Similarly, polyarylate copolymers containing carbonate linkages, i.e., poly(ester carbonates) display an excellent combination of properties.
Many processes have been described in the literature for the preparation of polyesters and poly(ester-carbonates). One such process reacts a dihydric phenol, a diacid and a diaryl carbonate. Specifically, this latter process involves the reaction of a dihydric phenol, iso- or terephthalic acids, or mixtures thereof, and a diaryl carbonate at temperatures of from about 260.degree. to about 350.degree. C. Optionally, a catalyst may be used.
The utilization of from about 10 to about 60 percent of a processing aid is beneficial in preparing polyesters and poly(ester carbonates) in that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control. Additionally, by using the processing aid, reaction times are relatively short so that the polyester or poly(ester carbonate) produced possess vastly improved color, as compared to those prepared under reaction conditions described in the prior art.
Finally, the use of the processing aid prevents the occurrence of undesirable "hot spots" which produce colored degradation products which contribute to the color and instability of the polyester or poly(ester carbonate).